Why do people complain about it giving horse back problems from sitting on it? It can't be worse than riding bareback and people do that all the time!

Depends. Most cross country, distance or competitive organizations prohibit the "treeless" just as they prohibit riding bareback. In spite of the treeless marketing that purports otherwise, treeless does not provide a structure to bridge the back.

Of your horse is young, or you are a normal sized person, do your horse a favor and lose the treeless. If you are a skinny and your horse is mature, enjoy.

If you are willing to spend for a saddle that is really kind to your horse, and is very comfortable for you, look at a flex panel.

Mike
If at first you don't succeed, figure out what is wrong, then FIX it!

I've ridden in a Torsion (demoed one for a week) and an Ansur. Like the Ansur better, I felt less "trapped" in it.

Do you use a special pad with it, a normal pad or no pad at all?
Skito pad. HOWEVER, I will say this seems to be very individual to the horse. Some never have any problems with a regular pad.

Are they very durable?
Not as durable as treed saddles. And by that I mean the leather is softer and more flexible than that used on treed saddles. Very comfortable, but on the flip side, it doesn't last as long. The craftsmanship on both of the saddles I rode in was good though.

Do you post? If you do, then does your horse get sore where the stirrup bars go?
I do post. I haven't noticed any pain with the Ansur...they have some sort of shock absorbing built into the saddle.

Now all of this is just my experience obviously. Some horses love these saddles. Some don't, you just have to try and see.